A gunman who announced online that he was planning to shoot two "pigs" in retaliation for the chokehold death of Eric Garner ambushed two police officers in a patrol car and shot them to death in broad daylight Saturday before running to a subway station and killing himself, authorities said.

The program started in 1992 at Arlington National Cemetery in Virginia and has spread to almost 550 locations nationally to honor deceased veterans and service members listed as missing in action during the holiday season.